B cell proliferation and B cell differentiation have been recognized as distinct components of the humoral immune response. Factors that regulate the immune response may do so by interacting with one or the other of these two B cell functions. Extending our investigation of the regulation of the immune response, we propose to characterize the mechanism of action of immunomodulators - LPS, macrophages, suppressor cells - in these terms. Specifically, we propose to (1) dissect mechanisms of LPS-induced B cell proliferation and differentiation, (2) define effects of LPS on the T cell-dependent production of antibody to SRBC in terms of their relation to proliferation of B cells, (3) determine the role of proliferation signals and differentiation signals in the T cell-independent production of antibody, and (4) characterize suppressor cells in terms of their effects on proliferation and differentiation of B cells.